Use XBlock 0.3
Add InheritanceKVS and standardize inherited attr patterns
Fix pylint and typos
Clarify repr by naming
Include mixins in advanced component class loading and preview modules
Merge pull request #881 from cpennington/dhm/editable_metadata

Include mixins in advanced component class loading and preview modules
Merge pull request #844 from edx/dhm/editable_metadata

refactoring of platform to xblock 0.3 w/ refactoring of inheritance in the platform to a consistent representation.